Cellulosic biomass is a significant renewable resource for the generation of sugars. Fermentation of these sugars can yield commercially valuable end-products, including biofuels and chemicals currently derived from petroleum. While the fermentation of simple sugars to ethanol is relatively straightforward, the efficient conversion of cellulosic biomass to fermentable sugars such as glucose is challenging. See, e.g., Ladisch et al., 1983, Enzyme Microb. Technol. 5:82. Cellulosic material may be pretreated chemically, mechanically or in other ways to increase the susceptibility of cellulose to hydrolysis. Such pretreatment may be followed by the enzymatic conversion of cellulose to glucose, cellobiose, cello-oligosaccharides and the like, using enzymes that specialize in breaking down the β-1-4 glycosidic bonds of cellulose. These enzymes are collectively referred to as “cellulases”.
Cellulases are divided into three sub-categories of enzymes: 1,4-β-D-glucan glucanohydrolase (“endoglucanase” or “EG”); 1,4-β-D-glucan cellobiohydrolase (“exoglucanase”, “cellobiohydrolase”, or “CBH”); and β-D-glucoside-glucohydrolase (“β-glucosidase”, “cellobiase” or “BG”). Endoglucanases randomly attack the interior parts and mainly the amorphous regions of cellulose, mostly yielding glucose, cellobiose, and cellotriose. Exoglucanases incrementally shorten the glucan molecules by binding to the glucan ends and releasing mainly cellobiose units from the ends of the cellulose polymer. β-glucosidases split the cellobiose, a water-soluble β-1,4-linked dimer of glucose, into two units of glucose. Efficient production of cellulases for use in processing cellulosic biomass would reduce costs and increase the efficiency of production of biofuels and other commercially valuable compounds.